1. Field of the Invention
The present invention generally relates to a mobile communications system which applies OFDM (Orthogonal Frequency Division Multiplexing) in downlink, and specifically relates to a base station apparatus, a user apparatus, and a method of mapping that effectively map a transmit signal to a sub-carrier.
2. Description of the Related Art
A communications scheme to succeed W-CDMA and HSDPA (i.e., LTE, or Long Term Evolution) is being considered by a W-CDMA standardization body 3GPP, and, as radio access schemes, OFDM for downlink and SC-FDMA (Single-Carrier Frequency Division Multiplexing) for uplink are being considered (see Non-patent document 1, for example).
OFDM is a scheme such that a frequency band is divided into multiple narrow frequency bands (sub-carriers) having data carried on the respective frequency bands to transmit the data, and the sub-carriers are densely lined up on the frequency bands without their interfering with one another while overlapping, which makes it possible to achieve high-speed transmission and improve the usage efficiency of the frequency.
SC-FDMA is a transmission scheme for dividing a frequency bandwidth, and transmitting using different frequency bands among multiple terminals to make it possible to reduce interference between terminals. SC-FDMA, which features a reduced variation in transmit power, makes it possible to achieve a wider coverage and reduced power consumption of a terminal.
In general, in mobile communications, there is a pilot signal for use in channel estimating and radio quality measuring, the pilot signal being called a downlink reference signal (DL RS) in LTE.
The downlink reference signal in LTE, which is expressed in a two-dimensional sequence, includes a two-dimensional orthogonal sequence and a two-dimensional pseudo random sequence. Mapping (a sub-carrier number) of a reference signal to a physical resource is expressed in the following equation (see Non-patent document 2, for example).
      k    =                  6        ⁢        m            +                        (                      v            +                                          f                hop                            ⁡                              (                                  ⌊                                      i                    /                    2                                    ⌋                                )                                              )                ⁢        mod        ⁢                                  ⁢        6                  l    =          {                                                  0              ⁢                                                                                                                                        if                  ⁢                                                                          ⁢                  n                                =                                                      0                    ⁢                                                                                  ⁢                    and                    ⁢                                                                                  ⁢                    p                                    =                  0                                            ,              1                                                                          1              ⁢                                                                                                                                        if                  ⁢                                                                          ⁢                  n                                =                                                      0                    ⁢                                                                                  ⁢                    and                    ⁢                                                                                  ⁢                    p                                    =                  2                                            ,              3                                                                                          N                symb                DL                            -              3                                                                                            if                  ⁢                                                                          ⁢                  n                                =                1                            ⁢                                                                                      
Here, k represents a sub-carrier number, l represents an OFDM symbol number, and i represents a slot number. Moreover,NsymbDL=7
m and n take the following integer values.
            m      =      0        ,    1    ,    …    ⁢                  ,                  ⌊                              N            BW            DL                                              N              BW              RB                        /            2                          ⌋            -      1            n    =          {                                                                                    0                  ,                  1                                                                                                                        if                      ⁢                                                                                          ⁢                      p                                        =                    0                                    ,                  1                                                                                    0                                                                                                                                                                                      if                            ⁢                                                                                                                  ⁢                            p                                                    =                          2                                                ,                                                  3                          ⁢                                                                                                          ⁢                          and                          ⁢                                                                                                          ⁢                          the                          ⁢                                                                                                          ⁢                          generic                          ⁢                                                                                                          ⁢                          frame                                                                                                                                                                        structure                        ⁢                                                                                                  ⁢                        is                        ⁢                                                                                                  ⁢                        used                                                                                                                                                                  0                  ,                  1                                                                                                                                                                                                      if                            ⁢                                                                                                                  ⁢                            p                                                    =                          2                                                ,                                                  3                          ⁢                                                                                                          ⁢                          and                          ⁢                                                                                                          ⁢                          the                          ⁢                                                                                                          ⁢                          alternative                                                                                                                                                                        frame                        ⁢                                                                                                  ⁢                        structure                        ⁢                                                                                                  ⁢                        is                        ⁢                                                                                                  ⁢                        used                                                                                                                          ⁢                                          ⁢                      N            BW            RB                          =        12            
Here,NBWDL,which is a number of sub-carriers in a system bandwidth, is 300 when the system bandwidth is 5 MHz, 600 when the system bandwidth is 10 MHz, and 1200 when the system bandwidth is 20 MHz. Moreover, p, which represents an antenna port number, is p=0 when only one antenna is used, while values p=0, 1, 2, 3 may be taken when four antennas may be used.
In the first Equation, the value v may be determined according to the following Equation.
  v  =      {                                        3            ⁢            n                                                              if              ⁢                                                          ⁢              p                        =            0                                                            3            +                          3              ⁢              n                                                                          if              ⁢                                                          ⁢              p                        =            1                                                            3            ⁢                          (                              i                ⁢                mod                ⁢                2                            )                                                                          if              ⁢                                                          ⁢              p                        =            2                                                            3            +                          3              ⁢                              (                                  i                  ⁢                  mod                  ⁢                  2                                )                                                                                        if              ⁢                                                          ⁢              p                        =            3                              Here, fhop(j), which is a cell-specific integer sequence, represents a hopping pattern which varies per sub-frame or slot of a downlink reference signal. In other words, varying fhop(j) per cell makes it possible to map, to a sub-carrier which varies per cell, the downlink reference signal. When the number of sub-frames in one radio frame is 10, the value of j becomes 0, 1, 2, . . . , 9. In other words, fhop(j) becomes a sequence having ten elements.
fhop(j) may be set with a non-time-dependent fixed value. When such a fixed value is set per cell, the downlink reference signal is mapped with a shift by a fixed value which varies per cell.
FIGS. 1A and 1B show exemplary mappings of the reference signal. A mapping (FIG. 1A) to a physical resource with an antenna port number set to No. 0 (p=0) and an element of fhop(j) always set to 0, and a mapping (FIG. 1B) to the physical resource with the antenna port number set to No. 0 (p=0) and an element of fhop(j) always set to 2 are shown. As shown, in the former, in a first OFDM symbol (l=0), a downlink reference signal is mapped to the kth (where k=6×j (j: integer not less than 0) sub-carrier. However, in the latter, in the first OFDM symbol (l=0), the downlink reference signal is mapped to the kth (k=6×j+2 (j: integer not less than 0) sub-carrier. In FIGS. 1A and 1B, a case such that the number of OFDM symbols per slot is 6 is shown. A similar operation is applied with respect to a mapping operation based on the above-described fhop(j) when the number of OFDM symbols per slot is 7 instead.
In FIGS. 1A and 1B, two examples are shown of when an element of fhop(j) is set to be always 0 and when an element of fhop(j) is set to be always 2. However, in an actual mobile communications system, a very large number of cells exist, so that the sequence fhop(j) in the respective cells must be set to differ from one another. In this case, it is difficult to set the sequence fhop(j) in the respective cells to differ from one another just by setting the element of fhop(j) to always take a fixed value.
For example, associating a cell group ID with the sequence fhop(j) is proposed as a method of setting the sequence fhop(j) for the respective cells (see Non-patent document 3, for example). In this case, for a cell group ID of 0 to 5, each element of fhop(j) will always take a fixed value, while, for a cell group ID of 6 to 169, fhop(j) will vary in its value from element to element. Then, 170 sequences fhop(j) associated with the cell group IDs will be set to differ from one another. Each element of the sequence fhop(j) will have as many varying values as possible, but will have partially the same values.
Non-patent document 1: 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006
Non-patent document 2: 3GPP TR 36.211 (V0.3.1), “Physical Channels and Modulation,” November 2006
Non-patent document 3: R1-070894, Cell-specific integer sequences for frequency positioning of DL RS, February, 2007